at
a. 表示"向,朝"
Don't laugh at others.不要嘲笑别人。
Please look at the blackboard.请看黑板。
b. 表示"处于……状态"
at breakfast 在吃早餐
at school在学校上
She is at work in the garden.她正在花园里干活。
c. 表示"在……方面"
He is good at English.他擅长英语。
They are busy at their homework.他们忙着做功课。